U.S. patent application number 13/059587 was filed with the patent office on 2011-06-23 for method and device for examining an exhaust gas sensor.
Invention is credited to Muammer Kilinc, Tim Walde.
Application Number | 20110146379 13/059587 |
Document ID | / |
Family ID | 41320364 |
Filed Date | 2011-06-23 |
United States Patent
Application |
20110146379 |
Kind Code |
A1 |
Kilinc; Muammer ; et
al. |
June 23, 2011 |
Method and Device for Examining an Exhaust Gas Sensor
Abstract
A first voltage (V1) is detected between an auxiliary pump
electrode and a reference electrode of an exhaust gas sensor, and a
target diagnosis value (SDIAG) is determined as a function of the
detected first voltage (V1). A measured current (Im) is detected
between a measurement electrode and a second main pump electrode of
the exhaust gas sensor, said measured current being set up as the
pump current by regulating a second voltage between the measurement
electrode and the reference electrode to a pre-defined voltage. An
actual diagnosis value (IDIAG) is determined as a function of the
detected measured current (Im). An error (ERR) of the exhaust gas
sensor is recognized depending on the target diagnosis value
(SDIAG) and the actual diagnosis value (IDIAG).
Inventors: |
Kilinc; Muammer;
(Regensburg, DE) ; Walde; Tim; (Regensburg,
DE) |
Family ID: |
41320364 |
Appl. No.: |
13/059587 |
Filed: |
August 18, 2009 |
PCT Filed: |
August 18, 2009 |
PCT NO: |
PCT/EP2009/060673 |
371 Date: |
March 9, 2011 |
Current U.S.
Class: |
73/23.31 |
Current CPC
Class: |
F01N 2560/026 20130101;
G01N 27/4175 20130101 |
Class at
Publication: |
73/23.31 |
International
Class: |
G01M 15/10 20060101
G01M015/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 18, 2008 |
DE |
10 2008 038 224.8 |
Claims
1-7. (canceled)
8. A method for testing an exhaust gas sensor having a first
chamber, a second chamber, a first diffusion barrier separating the
first and second chambers, and a second diffusion barrier forming a
gas inlet of the first chamber, the first chamber having first main
pump electrode, the second chamber having a measuring electrode and
an auxiliary pump electrode, and a second main pump electrode and a
reference electrode being disposed outside of the first and second
chambers, said method comprising: sensing a first voltage between
the auxiliary pump electrode and the reference electrode and
determining a set point diagnostic value as a function of the
sensed first voltage multiplied by a predefined factor and addition
of a predefined shift value; sensing a measuring current between
the measuring electrode and the second main pump electrode, the
measuring current being set as a pump current by adjusting a second
voltage between the measuring electrode and the reference electrode
to a predefined voltage, and determining an actual diagnostic value
as a function of the sensed measuring current; and detecting a
fault in the exhaust gas sensor as a function of the set point
diagnostic value and the actual diagnostic value.
9. The method of claim 8, wherein the detecting a fault comprises
detecting contamination of the exhaust gas sensor as the fault if
an oxygen concentration assigned to the actual diagnostic value in
the second chamber is lower by at least a predefined first absolute
value or factor than an oxygen concentration assigned to the set
point diagnostic value in the second chamber.
10. The method of claim 8, wherein the detecting a fault comprises
detecting detachment of a protective layer from the measuring
electrode as the fault if an oxygen concentration assigned to the
actual diagnostic value in the second chamber is larger by at least
a predefined second absolute value or factor than an oxygen
concentration assigned to the set point diagnostic value in the
second chamber.
11. The method of claim 8, further comprising sensing a further
pump current between the first main pump electrode and the second
main pump electrode, wherein the set point diagnostic value is
determined as a function of the sensed further pump current.
12. The method of claim 11, wherein at least one of the predefined
factor and the predefined shift value are selected as a function of
the sensed further pump current.
13. A device for testing an exhaust gas sensor, the exhaust gas
sensor having a first chamber, a second chamber, a first diffusion
barrier separating the first and second chambers, and a second
diffusion barrier forming a gas inlet of the first chamber, the
first chamber having a first main pump electrode and the second
chamber has a measuring electrode and an auxiliary pump electrode,
and a second main electrode and a reference electrode being
disposed outside of the first and second chambers, the device being
configured to sense a first voltage between the auxiliary pump
electrode and the reference electrode and determine a set point
diagnostic value as a function of the sensed first voltage
multiplied by a predefined factor and added to a predefined shift
value, sense a measuring current between the measuring electrode
and the second main pump electrode, the measuring current being set
as a pump current by a second voltage between the measuring current
being set as a pump current by a second voltage between the
measuring electrode and the reference electrode to a predefined
voltage, and determine an actual diagnostic value as a function of
the sensed measuring current, and detect a fault in the exhaust gas
sensor as a function of the set point diagnostic value and the
actual diagnostic value.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a U.S. national stage of application No.
PCT/EP2009/060673, filed on Aug. 18, 2009. Priority is claimed on
German Application No. 10 2008 038 224.8, filed Aug. 18, 2008 the
content of which is incorporated here by reference.
BACKGROUND OF THE INVENTION
[0002] 1.Field of the Invention
[0003] The invention relates to a method and to a device for
testing an exhaust gas sensor and, in particular, an NOx sensor for
motor vehicles with a diesel internal combustion engine.
[0004] 2. Description of the Prior Art
[0005] Due to the increasingly strict legal requirements in terms
of permissible emissions of pollutants in motor vehicles in which
internal combustion engines are arranged there is a need to keep
the emissions of pollutants during operation of the internal
combustion engine as low as possible. This can be achieved, for
example, by reducing the emissions of pollutants which are produced
during the combustion of the air/fuel mixture in the respective
cylinder of the internal combustion engine. On the other hand,
exhaust gas post-treatment systems are used in internal combustion
engines, said exhaust gas post-treatment systems converting the
emissions of pollutants which are produced during the combustion
process of the air/fuel mixture in the respective cylinder into
harmless substances. For this purpose, catalytic converters are
used which convert carbon monoxide, hydrocarbons and nitrogen
oxides into harmless substances. Both the aimed-at influencing of
the production of desired way over a long service life and faults
are reliably detected.
[0006] The manual "Handbuch Verbrennungsmotor" [Internal Combustion
Engine Manual], published by Richard van Basshuysen/Fred Schafer,
2.sup.nd Edition, June 2002, the emissions of pollutants during the
combustion and the conversion of the components of the pollutants
with a high degree of efficiency by means of an exhaust gas
catalytic converter require a very precisely set air/fuel ratio in
the respective cylinder. NOx sensors are used to determine the
nitrogen oxide content in the exhaust gas. In this context it is
necessary to ensure that the components of the exhaust gas
post-treatment system also function in the
[0007] Friedrich Vieweg & Sohn Verlagsgesellschaft mbH
Braunschweig/Wiesbaden, pages 589 to 590, discloses an NOx sensor
based on a ZrO.sub.2 ceramic, which NOx sensor has two chambers. In
the first chamber, a constant partial pressure of the oxygen
contained in the exhaust gas is brought about by applying a pump
current. The pump current is inverted proportional to the air/fuel
ratio. In the second chamber, the nitrogen oxide contained in the
exhaust gas is decomposed by applying a further current. This
current is proportional to the nitrogen oxide content in the
exhaust gas and forms the measuring signal of the NOx sensor.
[0008] Document DE 199 07 947 B4 discloses a circuit for an NOx
measuring pickup which has a first measuring cell and a second
measuring cell which is connected to the first measuring cell. The
measuring cells are located in a solid electrolyte. The circuit has
a first circuit arrangement which sets a different oxygen
concentration in the first measuring cell than in the gas to be
measured by tapping a first Nernst voltage which serves as a first
reference variable. A second circuit arrangement sets a different
oxygen concentration in the second measuring cell than in the first
measuring cell by tapping a second Nernst voltage which serves as a
second reference variable. A third circuit arrangement drives a
pump current composed of oxygen ions which originate from NOx out
of the second measuring cell by tapping a third Nernst voltage
which serves as a third reference variable.
[0009] The exhaust gas sensor may be contaminated due to components
in the exhaust gas. This may make it necessary to carry out
diagnostics of the exhaust gas sensor during the ongoing operation
of the internal combustion engine.
SUMMARY OF THE INVENTION
[0010] The object of the invention is to provide a method and a
device for testing an exhaust gas sensor which permit reliable
detection of faults in the exhaust gas sensor.
[0011] The invention is defined by a method and a corresponding
device for testing an exhaust gas sensor. The exhaust gas sensor
has a first chamber and a second chamber which are separated from
one another by a diffusion barrier. The exhaust gas sensor also has
a further diffusion barrier which forms a gas inlet of the first
chamber. The first chamber has a first main pump electrode, and the
second chamber has a measuring electrode and an auxiliary pump
electrode. The exhaust gas sensor also has a second main pump
electrode and a reference electrode outside the first and second
chambers. A first voltage is sensed between the auxiliary pump
electrode and the reference electrode. A set point diagnostic value
is determined as a function of the sensed first voltage. A
measuring current is sensed between the measuring electrode and the
second main pump electrode. The measuring current is set as a pump
current by adjusting a second voltage between the measuring
electrode and the reference electrode to a predefined voltage. An
actual diagnostic value is determined as a function of the sensed
measuring current. A fault in the exhaust gas sensor is detected as
a function of the set point diagnostic value and the actual
diagnostic value.
[0012] The advantage is that faults in the exhaust gas sensor, in
particular contamination of the exhaust gas sensor and/or
detachment of a protective layer from the measuring electrode, can
be reliably detected. In addition, the testing of the exhaust gas
sensor can advantageously also be reliably carried out in diesel
internal combustion engines.
[0013] The exhaust gas sensor is embodied, in particular, as an NOx
sensor. However, the exhaust gas sensor can likewise be designed to
sense one or more other components of the exhaust gas. The first
and second voltages are, in particular, Nernst voltages. The first
voltage is, in particular, representative of an oxygen content in
the second chamber. In particular, for this purpose a predefined
first pump current is set between the auxiliary pump electrode and
the main pump electrode, and the resulting first voltage is then
dependent on the oxygen content in the second chamber.
[0014] As a result of the adjustment of the second voltage to the
predefined voltage, essentially a stoichiometric gas mixture is
formed in the surroundings of the measuring electrode, that is to
say the oxygen from the surroundings of the measuring electrode is
pumped away by the measuring current in such a way that the
surroundings of the measuring electrode are essentially free of
oxygen. The predefined voltage is, for example, approximately
between 400 and 450 millivolts. However, the predefined voltage can
also have a different value.
[0015] The testing of the exhaust gas sensor is, in particular,
carried out when at least one predefined diagnostic condition
applies. The at least one predefined diagnostic condition can, in
particular, comprise a predefined operating state, for example
overrun conditions, or generally an operating state with an
essentially nonvariable load and as a result an essentially
nonvariable and predefined oxygen content in the exhaust gas. The
at least one predefined diagnostic condition can also comprise, in
particular, a low NOx measured value of the exhaust gas sensor
which is below a predefined NOx threshold value.
[0016] In one advantageous refinement, contamination of the exhaust
gas sensor is detected as a fault if an oxygen concentration which
is assigned to the actual diagnostic value in the second chamber is
lower by at least a predefined first absolute value or factor than
an oxygen concentration which is assigned to the set point
diagnostic value in the second chamber. In particular, the
contamination of the exhaust gas sensor is detected as a fault if
the actual diagnostic value is smaller than the set point
diagnostic value by at least a predefined lower threshold value.
This has the advantage that the contamination of the exhaust gas
sensor can be easily and reliably detected.
[0017] In a further embodiment, detachment of a protective layer
from the measuring electrode is detected as a fault if an oxygen
concentration which is assigned to the actual diagnostic value in
the second chamber is larger by at least a predefined second
absolute value or factor than an oxygen concentration which is
assigned to the set point diagnostic value in the second chamber.
In particular, the detachment of the protective layer is detected
as a fault if the actual diagnostic value is larger than the set
point diagnostic value by at least a predefined upper threshold
value. This has the advantage that the detachment of the protective
layer can be easily and reliably detected.
[0018] In a further embodiment, the set point diagnostic value is
determined as a function of multiplication of the first voltage by
a predefined factor and addition of a predefined shift value. The
predefined factor and the predefined shift value are, in
particular, predefined individually for the exhaust gas sensor or
for a type or a design of the exhaust gas sensor and are
determined, for example, as a function of calibration of the
exhaust gas sensor, which calibration is carried out, in
particular, at the premises of a manufacturer of the exhaust gas
sensor within the scope of the manufacture of the exhaust gas
sensor. The advantage is that this is easy. In addition, the set
point diagnostic value can be determined reliably in this way.
[0019] In a further embodiment, a further pump current is sensed
between the first main pump electrode and the second main pump
electrode. The set point diagnostic value is determined as a
function of the sensed further pump current. The advantage is that
the set point diagnostic value can be particularly reliably
determined in this way. In addition, the testing of the exhaust gas
sensor can be carried out even if the current oxygen content in the
exhaust gas is unknown. The testing of the exhaust gas sensor can
therefore be reliably carried out at different respective
prevailing exhaust gas lambda values and can be carried out, in
particular, at all prevailing exhaust gas lambda values. As a
result, the testing of the exhaust gas sensor is also particularly
suitable for diesel internal combustion engines in which exhaust
gas lambda values can differ significantly from one, and under
certain circumstances fluctuate to a high degree. The further pump
current is preferably representative of an oxygen concentration in
the exhaust gas and is, in particular, proportional to the oxygen
concentration in the exhaust gas.
[0020] In this context it is advantageous if the set point
diagnostic value is determined as a function of multiplication of
the first voltage by a predefined factor and addition of a
predefined shift value. The predefined factor and/or the predefined
shift value are selected as a function of the sensed further pump
current. The predefined factor and the predefined shift value are,
in particular, predefined individually for the exhaust gas sensor
or for a type or a design of the exhaust gas sensor, for a
respectively associated value or value range of the further
voltage. The predefined factor, the predefined shift value and/or
the respectively associated value or value range of the further
pump current are determined, for example, as a function of
calibration of the exhaust gas sensor. The calibration is carried
out, for example, at the premises of a manufacturer of the exhaust
gas sensor within the scope of the manufacture of the exhaust gas
sensor. The advantage is that this is very easy and the diagnosis
of the exhaust gas sensor can be carried out reliably in this way.
The set point diagnostic value can be determined particularly
reliably.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Exemplary embodiments of the invention are explained below
with reference to the schematic drawings, in which:
[0022] FIG. 1 shows an internal combustion engine with a control
device,
[0023] FIG. 2 shows a cross section through an exhaust gas sensor
and a measuring circuit,
[0024] FIG. 3 shows a diagram with diagnostic values which are
plotted against a first voltage, and
[0025] FIG. 4 shows a flowchart.
[0026] Elements with the same design or function are provided with
the same reference symbols in all the figures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] FIG. 1 shows an internal combustion engine having an intake
section 10, an engine block 12, a cylinder head 13 and an exhaust
section 14. The intake section 10 preferably comprises a throttle
valve 15, a collector 16 and an intake manifold 17. The intake
manifold 17 is extended to a cylinder Z1 in the inlet duct into a
combustion chamber 26 of the engine block 12. The engine block 12
comprises a crankshaft 18 which is coupled via a connecting rod 20
to a piston 21 of the cylinder Z1.
[0028] The cylinder head 13 comprises a valve drive with a gas
inlet valve 22 and a gas outlet valve 24. The cylinder head 13 also
comprises an injection valve 28, and a spark plug 30 if the
internal combustion engine is embodied, for example, as a petrol
internal combustion engine. Alternatively, the injection valve 28
can also be arranged in the intake manifold 17.
[0029] An exhaust gas catalytic converter 32 is arranged in the
exhaust section 14. In addition, a catalytic converter 34 for
reducing NOx is arranged in the exhaust section.
[0030] The internal combustion engine is also assigned a control
device 35 which is assigned sensors which sense various measurement
variables and respectively determine the value of the measurement
variables. The control device 35 is designed to determine
manipulated variables as a function of at least one of the
measurement variables, which manipulated variables can then be
converted into one or more actuating signals for controlling
actuator elements by means of corresponding actuator drives. The
control device 35 can also be embodied as a device for testing an
exhaust gas sensor SENS or comprise such a device.
[0031] The actuator elements are, for example, the throttle valve
15, the gas inlet valve 22 and gas outlet valve 24, the injection
valve 28 or the spark plug 30.
[0032] The sensors comprise, for example, a pedal position sensor
36 which senses a position of an accelerator pedal 38. Furthermore,
the internal combustion engine has an air mass flow rate sensor 40
which is arranged upstream of the throttle valve 15 and senses an
air mass flow there. A temperature sensor 42 upstream of the
throttle valve 15 senses an intake air temperature. An intake
manifold pressure sensor 44 downstream of the throttle valve 15 is
arranged in the collector 16 and senses an intake manifold pressure
in the collector 16. Furthermore, the internal compression engine
comprises a crankshaft angle sensor 46 which senses a crankshaft
angle to which a rotational speed of the internal combustion engine
can be assigned.
[0033] An exhaust gas probe 50 is arranged upstream of the exhaust
gas catalytic converter 32, which exhaust gas probe 50 senses a
residual oxygen content of the exhaust gas and the measurement
signal of said exhaust gas probe 50 is characteristic of the
air/fuel ratio in the combustion chamber 26 of the cylinder Z1 and
upstream of the exhaust gas probe 50 before oxidation of the fuel.
In addition, a lambda probe 52 is provided which is arranged
downstream of the exhaust gas catalytic converter 32 and which
senses a residual oxygen content of the exhaust gas and the
measurement signal of said lambda probe 52 is characteristic of the
air/fuel ratio in the combustion chamber 26 of the cylinder Z1 and
upstream of the lambda probe 52.
[0034] The exhaust gas probe 50 and the lambda probe 52 are
preferably binary lambda probes. The exhaust gas probe 50 and/or
the lambda probe 52 can, however, basically be embodied
individually or else together as linear lambda probes.
[0035] An exhaust gas probe 53 and the exhaust gas sensor SENS are
arranged downstream of the exhaust gas catalytic converter 32. The
exhaust gas probe 53 senses a residual oxygen content of the
exhaust gas and the measurement signal of said exhaust gas probe 53
is characteristic of the air/fuel ratio upstream of the exhaust gas
probe 53. The exhaust gas sensor SENS senses, in particular, an NOx
concentration of the exhaust gas upstream of the exhaust gas probe
53. In the text which follows, the exhaust gas sensor SENS and a
diagnostic method are illustrated with respect to NOx by way of
example. However, it is correspondingly also possible for one or
more other components of the exhaust gas to be sensed by means of
the exhaust gas sensor, and diagnostics can be performed on the
exhaust gas sensor SENS with respect to this component or these
components.
[0036] The exhaust gas sensors SENS preferably comprises the
exhaust gas probe 53. This has the disadvantage that just a single
sensor has to be made available for sensing the NOx concentration
and the residual oxygen content of the exhaust gas. The exhaust gas
sensor SENS is preferably designed to output a binary lambda
signal. This is advantageous since the binary lambda signal is very
sensitive to the residual oxygen content of the exhaust gas.
However, the exhaust gas sensor SENS can basically comprise a
linear lambda probe.
[0037] Depending on the embodiment of the invention, any desired
subset of the specified sensors may be present, or additional
sensors may also be present.
[0038] In addition to the cylinder Z1, further cylinders Z2, Z3, Z4
are preferably also provided, corresponding actuator elements and,
if appropriate sensors, being likewise assigned to said
cylinders.
[0039] The internal combustion engine is preferably embodied as a
diesel internal combustion engine. However, the internal combustion
engine can also be embodied in a different way, for example as a
petrol internal combustion engine.
[0040] The device for testing the exhaust gas sensor SENS is
preferably arranged in the exhaust gas sensor SENS itself. A system
comprising the exhaust gas sensor SENS and the device for testing
the exhaust gas sensor SENS can be of such particularly compact
design, and can, in particular be independent of the control device
35 of the internal combustion engine.
[0041] FIG. 2 shows the exhaust gas sensor SENS in a cross section
and a measuring circuit for operating the exhaust gas sensor SENS.
The exhaust gas sensor SENS comprises a solid electrolyte E, which
is preferably formed from zirconium dioxide ZrO.sub.2, and in which
a heater H is arranged. An air duct with an air inlet LE for
feeding in air from the surroundings is formed in the solid
electrolyte E. In addition, a first chamber MK1 and a second
chamber MK2 are formed in the solid electrolyte E. The first
chamber MK1 comprises a first main pump electrode HP1. The second
chamber MK2 comprises an auxiliary pump electrode M1 and a
measuring electrode M2. In addition, a second main pump electrode
HP2 is arranged on the outside of the solid electrolyte E, and a
reference electrode REF is arranged in the air duct. The first
chamber MK1 and the second chamber MK2 are separated from one
another by a diffusion barrier DB2. A further diffusion barrier DB1
forms a gas inlet for a gas whose NOx content is to be determined.
The gas is formed, in particular, by exhaust gas of the internal
combustion engine. The diffusion barrier DB2 and the further
diffusion barrier DB1 are, in particular, permeable to nitrogen
oxides NOx. In general, the diffusion barrier DB2 and the further
diffusion barrier DB1 are also permeable to oxygen and/or other
components of the gas, and are, in particular, permeable to the
exhaust gas of the internal combustion engine.
[0042] The measuring circuit comprises a first regulator C1, a
first voltage-controlled power source UI1 and preferably a first
conditioning device K1. The auxiliary pump electrode M1 and the
reference electrode REF of the exhaust gas sensor SENS are coupled
to an input of the first regulator C1 via the first conditioning
device K1 for the purpose of sensing a first voltage V1 between the
auxiliary pump electrode M1 and the reference electrode REF. On the
output side, the first regulator C1 is coupled via the first
voltage-controlled power source UI1 to the second main pump
electrode HP2 and to the auxiliary pump electrode M1 for the
purpose of driving a first pump current IP1 between the second main
pump electrode HP2 and the auxiliary pump electrode M1. The first
regulator C1 is, in particular, designed to regulate the first pump
current IP1 as a function of the first voltage V1. The first
regulator C1 is also preferably embodied in such a way that during
the testing of the exhaust gas sensor SENS the first pump current
IP1 is set or adjusted to a predefined pump current which is, in
particular, predefined as a constant.
[0043] The measuring circuit also comprises a second regulator C2,
a second voltage-controlled power source UI2 and preferably a
second conditioning device K2. The measuring electrode M2 and the
reference electrode REF of the exhaust gas sensor SENS are coupled
to an input of the second regulator C2 via the second conditioning
device K2 for the purpose of sensing a second voltage V2 between
the measuring electrode M2 and the reference electrode REF. On the
output side, the second regulator C2 is coupled via the second
voltage-controlled power source UI2 to the second main pump
electrode HP2 and to the measuring electrode M2 for the purpose of
driving a second pump current which forms a measuring current Im,
between the second main pump electrode HP2 and the measuring
electrode M2. The second regulator C2 is, in particular, designed
to regulate the second pump current, that is to say the measuring
current Im, as a function of the second voltage V2. The second
regulator C2 is, in particular, designed to regulate the measuring
current Im in such a way that the second voltage V2 corresponds to
a predefined voltage. The predefined voltage is, in particular,
predefined in such a way that a stoichiometric gas mixture is
formed in the surroundings of the measuring electrode M2, that is
to say the surroundings of the measuring electrode M2 are
essentially free of oxygen. The predefined voltage can be, for
example, approximately between 400 and 450 millivolts. However, the
predefined voltage can also be higher than 450 millivolts or lower
than 400 millivolts.
[0044] In addition, the measuring circuit comprises a further
regulator C0, a further voltage controlled power source UI0 and
preferably a further conditioning device K0. The first main pump
electrode HP1 and the reference electrode REF of the exhaust gas
sensor SENS are coupled via the further conditioning device K0 to
an input of the further regulator C0 for the purpose of sensing a
further voltage V0 between the first main pump electrode HP1 and
the reference electrode REF. On the output side, the further
regulator C0 is coupled via the further voltage-controlled power
source UI0 to the first and second main pump electrodes HP1, HP2
for the purpose of driving a further pump current IP0 between the
first and second main pump electrodes HP1, HP2. The further
regulator C0 is, in particular, designed to regulate the further
pump current IPO as a function of the further voltage V0. The first
chamber MK1 forms, together with the first and second main pump
electrodes HP1, HP2 and the reference electrode REF, in particular
a binary lambda probe and can represent, in particular, the exhaust
gas probe 53.
[0045] The pump currents, that is to say the first, second and
further pump currents IP1, IP0, cause oxygen to be transported
through the solid electrolyte E and into the respective chamber or
out of the respective chamber, said solid electrolyte E being
heated to a suitable temperature for this by the heater H. The
voltages, that is to say the first, second and further voltages V1,
V2, V0 are, in particular, Nernst voltages and are dependent on the
respective oxygen concentrations in the first chamber MK1, the
second chamber MK2 and the surrounding air at the reference
electrode REF. In order to test the exhaust gas sensor SENS, the
first and further pump currents IP1, IP0 are preferably set or
adjusted in such a way that a predefined oxygen concentration,
which is in particular, greater than zero parts per million, occurs
in the second chamber MK2. The oxygen which is located in the
second chamber MK2 acts on the first voltage V1, and, in the case
of a sufficiently functionally capable exhaust gas sensor SENS,
said oxygen acts on the second pump current, that is to say the
measuring current Im.
[0046] For example, the measuring electrode M2 is provided with a
protective layer which is permeable to nitrogen oxides NOx and
oxygen. However, contamination of the measuring electrode M2 and,
in particular, of the protective layer may adversely affect the
sensitivity of the measuring electrode M2 to oxygen or the
permeability of he protective layer to oxygen. The contamination of
the exhaust gas sensor SENS causes the measuring current Im to
generally deviate from the measuring current Im which is expected
for the fault-free exhaust gas sensor SENS. The measuring current
Im of the contaminated exhaust gas sensor SENS is, in particular,
too low.
[0047] However, the protective layer may, for example, become
detached from the measuring electrode M2. As a result, it is
generally possible for more oxygen to arrive at the measuring
electrode M2 than when the protective layer is intact. As a result
of the detachment of the protective layer from the measuring
electrode M2, the measuring current Im generally deviates from the
measuring current Im which is expected for the fault-free exhaust
gas sensor SENS. The measuring current Im is in particular too
large when the protective layer is detached.
[0048] In order to carry out the diagnostics, that is to say the
testing of the exhaust gas sensor SENS, a program can be stored in
a program memory of the control device 35 and run while the
internal combustion engine is operating. FIG. 4 shows a flow chart
of a method for testing the exhaust gas sensor SENS and the
program. FIG. 3 shows a diagram with diagnostic values DIAG.
[0049] The method starts at a step S1. A step S2 can be provided
for testing whether at least one predefined diagnostic condition B
applies. The at least one predefined diagnostic condition B can
comprise, in particular, a predefined operating state of the
internal combustion engine, for example overrun conditions, or
generally an operating state with an essentially nonvariable load
and as a result an essentially nonvariable and predefined oxygen
concentration in the exhaust gas. The at least one predefined
diagnostic condition can also comprise, in particular, a small NOx
measured value of the exhaust gas sensor SENS which is below a
predefined NOx threshold value. The predefined NOx threshold value
can, for example, be approximately 50 parts per million. However,
the predefined NOx threshold value can likewise be higher or lower
than 50 parts per million. It is also possible for further or other
diagnostic conditions B to be provided. Preferably, the predefined
operational state lasts for the duration of the diagnostics
performed on the exhaust gas sensor SENS. The duration is, for
example, approximately five seconds but can also be longer or
shorter.
[0050] A step S3 can be provided in which the further pump current
IPO is sensed. The further pump current IPO is dependent on the
oxygen concentration in the first chamber MK1 and is preferably
representative of the oxygen concentration in the first chamber
MK1. The further pump current IPO is preferably sensed at the start
of the diagnostics. The further pump current IP0 can, in
particular, also represent an oxygen concentration of the exhaust
gas. It is possible to provide for this purpose that the further
voltage V0 be adjusted to a predefined value. The step S3 can, in
particular, be dispensed with if the testing of the exhaust gas
sensor SENS is carried out in an operating state of the internal
combustion engine for which the oxygen concentration in the exhaust
gas is known. This can, for example, be the case during overrun
conditions.
[0051] In a step S4, the first pump current IP1 and the further
pump current IP0 are predefined. The first pump current IP1 is
preferably predefined reduced in absolute value compared to a
normal sensor operating mode of the exhaust gas sensor SENS outside
the diagnostics. In a step S5, the first voltage V1 which is set is
sensed. The first voltage V1 is dependent on the oxygen
concentration in the second chamber MK2 and is preferably
representative of the oxygen concentration in the second chamber
MK2. The associated oxygen concentration in the second chamber MK2
is preferably assigned to the sensed first voltage V1.
[0052] In general, the oxygen concentration, which can be
determined as a function of the first voltage V1, is only a rough
approximation compared to the oxygen concentration which can in
principle be determined as a function of the measuring current Im.
However, said oxygen concentration is insensitive to contamination
and detachment of the protective layer of the measuring electrode
M2.
[0053] In a step S6, a set point diagnostic value SDIAG is
determined as a function of the first voltage V1. If the further
pump current IPO has been sensed in the step S3, in the step S6 the
set point diagnostic value SDIAG is preferably determined as a
function of the first voltage V1 and the further pump current IPO
which was sensed in step S3. For example, in a step S6a a
predefined factor F and/or a predefined shift value O are selected
as a function of the further pump current IPO. The respectively
associated predefined factor F and/or predefined shift value O,
from which a selection can be made as a function of the further
pump current IP0 sensed in the step S3, are preferably predefined
and stored for various values or value ranges of the further pump
current IP0. If the further pump current IPO has not been sensed,
the predefined factor F and the predefined shift value are
preferably permanently predefined independently of the further pump
current IP0, being predefined in particular as constants. In a step
S6b, the set point diagnostic value SDIAG is determined, for
example, as a function of the predefined factor F, the first
voltage V1 and the predefined shift value O, for example as a
function of multiplication of the predefined factor F by the first
voltage V1 and addition of the predefined shift value O:
SDIAG=f(F*V1+O), and in particular: SDIAG=F*V1+O.
[0054] In FIG. 3, the set point diagnostic value SDIAG is
represented as a function of the first voltage V1 as a
characteristic curve which is, in particular, a straight line. The
predefined factor F and the predefined shift value O are preferably
determined experimentally and in particular by calibration. FIG. 3
illustrates, by way of example, three calibration values KAL, which
have been sensed, for example, during the calibration of the
exhaust gas sensor SENS. The calibration is preferably carried out
in a new state of the exhaust gas sensor SENS and is preferably
carried out within the scope of the manufacture of the exhaust gas
sensor SENS. The characteristic curve results, for example, as a
regression straight line as a function of the calibration values
KAL, on which straight line the predefined factor F can be
determined easily as a function of the gradient thereof, and the
predefined shift value O can easily be determined. The calibration
values KAL which each form the basis for such a characteristic
curve are preferably sensed for a predefined oxygen concentration
in a gas which is used for the calibration, if appropriate instead
of the exhaust gas of the internal combustion engine. By varying
the predefined oxygen concentration in the gas it is possible for
various characteristic curves with different predefined factors F
and/or predefined shift values O to be formed between which,
according to step S6a, it is possible to select as a function of
the further pump current IPO. In this way a high degree of accuracy
of the set point diagnostic value SDIAG is possible.
[0055] In a step S7, the measuring current Im is sensed. In a step
S8, an actual diagnostic value IDIAG which is associated with the
measuring current Im is determined. An associated oxygen
concentration at the measuring electrode M2 in the second chamber
MK2 which, however, under certain circumstances, does not
correspond to the actual oxygen concentration owing, for example,
to contamination or detachment of the protective layer, is
preferably assigned to the measuring current Im or the diagnostic
value IDIAG. In a step S9, it is tested whether deviation of the
oxygen concentration assigned to the actual diagnostic value IDIAG
from the oxygen concentration assigned to the set point diagnostic
value SDIAG occurs, by testing, for example, whether a deviation of
the actual diagnostic value IDIAG from the set point diagnostic
value SDIAG exceeds a predefined threshold value. If this is not
the case, the exhaust gas sensor SENS is detected in a step S10 as
free of faults and the testing of the exhaust gas sensor SENS is
ended in a step S11.
[0056] However, if it is detected in the step S9, for example, that
the actual diagnostic value, IDIAG is lower than the set point
diagnostic value SDIAG by more than a predefined lower threshold
value USW, that is to say that the oxygen concentration which is
assigned to the actual diagnostic value IDIAG is too low compared
to the oxygen concentration assigned to the set point diagnostic
value SDIAG, in a step S12 a first fault ERR1 is detected as a
fault ERR of the exhaust gas sensor SENS. The first fault ERR1
corresponds, in particular, to the contamination of the exhaust gas
sensors SENS. If appropriate, a corresponding entry is made in a
fault memory and/or the fault ERR is signaled. The testing of the
exhaust gas sensor SENS is ended in the step S11.
[0057] If it is detected in the step S9 that, for example, the
actual diagnostic value IDIAG is higher than the set point
diagnostic value SDIAG by more than a predefined upper threshold
value OSW, that is to say that the oxygen concentration assigned to
the actual diagnostic value IDIAG is too high compared to the
oxygen concentration assigned to the set point diagnostic value
SDIAG, in a step S13 a second fault ERR2 is detected as a fault ERR
of the exhaust gas sensor SENS. The first fault ERR2 corresponds,
in particular, to the detachment of the protective layer from the
measuring electrode M2 of the exhaust gas sensor SENS. If
appropriate, a corresponding entry is made in the fault memory
and/or the fault ERR is signaled. The testing of the exhaust gas
sensor SENS is ended in the step S11.
[0058] The relationship between diagnostic values DIAG and the
assigned oxygen concentration can alternatively also be predefined
inversely, that is to say, for example, the first fault ERR1 is
detected if it is detected in the step S9 that the actual
diagnostic value IDIAG is higher than the set point diagnostic
value SDIAG by more than the predefined lower threshold value USW,
which means that the oxygen concentration assigned to the actual
diagnostic value IDIAG is too low compared to the oxygen
concentration assigned to the set point diagnostic value SDIAG,
and, for example, the second fault ERR2 is detected if it is
detected in the step S9 that the actual diagnostic value IDIAG is
lower than the set point diagnostic value SDIAG by more than the
predefined upper threshold value OSW, which means that the oxygen
concentration assigned to the actual diagnostic value IDIAG is too
high compared to the oxygen concentration assigned to the set point
diagnostic value SDIAG.
* * * * *